This invention relates to improvements in voltage regulating apparatus. More particularly, it is directed to a voltage regulator which is particularly well suited for regulating the projection lamp voltage in a photographic printing apparatus.
Professional photographers, as well as many hobbyists, have come to realize that while regulation of the lamp voltage in a photographic enlarger is merely desirable in making individual black-and-white prints, it is almost an absolute necessity in producing color prints and in consistently producing stabilized black-and-white prints. Such a realization probably came as no surprise to the professional photographer, for manufacturers and suppliers of photographic equipment and color chemistry had, for years, cautioned photographers about the necessity of stabilizing the lamp voltage in order to avoid waste of both time and expensive materials. It can easily be shown that a variation in the line voltage of only five volts results in a change in lamp intensity by about 15% (not to mention the substantial variation in the color temperature or spectral content of the lamp it produces). Such a variation can spell the difference between high quality and mediocre results.
Variations in line voltage are generally of two types: (1) those which are short-lived, momentary surges or drops in line voltage; and (2) those which occur over relatively long periods of time, (e.g. hours or days). The former type of variations are sometimes referred to as electrical transients. They are produced, for example, during the start-up high current-drawing appliances, such as refrigerators, dryers and furnaces. The latter type of variations results from gradual changes in the total load on a power company's generator or from cutbacks in power initiated by the power company.
Heretofore, photographers had a choice between two types of general purpose voltage regulators for maintaining a substantially constant lamp voltage. The type most frequently used, and the one which has become the industry standard, is the saturable-core, constant voltage transformer. Properly used, it regulates voltage quite well. Its primary disadvantages, however, are its size, weight, and cost, each of such factors increasing in direct proportion to the power rating of the transformer. This type of regulator is gradually seeing less use because the lamp power in color enlargers has increased to 500 watts and beyond.
Another disadvantage of the saturable core regulator, one not generally recognized, is that the degree of regulation is dependent upon the load being drawn. To achieve the specified regulation, it is necessary to match the load to the regulator. This precludes purchasing a regulator which is sized to meet the largest anticipated need and using it for smaller loads with the specified regulation.
A further disadvantage of the saturable core regulator is the sixty cycle hum associated therewith. Many photographers find this hum virtually unbearable in a small, confining darkroom. Further, care must be taken to keep the vibration associated with the hum away from the enlarger.
The second available choice in voltage regulating equipment is the direct current type. While well designed regulators of this type provide adequate regulation, the D.C. output is a serious disadvantage. Direct current regulators cannot be used with a transformer to change the voltage applied to the lamp. This is a serious disadvantage for those who wish to use low voltage quartzhalogen lamps, or to those who want to increase the lamp voltage to enhance the lamp intensity for extra large prints. Another problem is that sooner or later someone will accidentally plug equipment other than lamps (e.g. timers, blowers, etc.) into the D.C. regulator. Many photographers have learned to their dismay that A.C. equipment in their darkrooms is seriously damaged or ruined by D.C. voltage.